This invention relates to electrostatographic production of copies and method and apparatus for the production of same.
In this specification, the expression "electrostatographic reproduction machine" refers to a machine for producing one or more prints or copies from at least one electrostatic latent image. The expression "electrostatographic member" refers to a member for producing and utilizing an electrostatic latent image.
The production of an electrostatic latent image may be carried out in various ways as is well known. The basic and most conventional electrostatographic process or method is described in U.S. Pat. No. 2,297,691. This method involves producing a uniform electrostatic charge on a photoconductive insulating layer. In practice, it is possible for the insulating layer to have a protective overlayer or other overlayer known in the art of xerography. The charged layer is exposed to imaging radiation (especially light) to discharge selectively the photoconductive layer to form the electrostatic latent image. The latent image may then be developed in any known way. Examples of known development methods, for example, are "cascade development" described in U.S. Pat. No. 2,221,776; and "magnetic brush development" described in U.S. Pat. No. 2,874,063. Another example of a known development method is a liquid development method described in U.S. Pat. No. 3,084,043. In this method, development is carried out with a polar liquid developer. Such a developer is stable, i.e. it will respond to an electrostatic field as a homogeneous unit without separation of the components of the liquid developer. As described in U.S. Pat. No. 3,084,043, the polar liquid developer is applied by a rotatable member having a plurality of raised portions defining a substantially regular patterned surface and a plurality of portions depressed or sunken below the raised portions. The liquid developer is present in the depressed portions and is doctored by a doctor blade.
In present electrostatographic machines the photoconductive layer is charged as far as practically possible uniformly along its length. In such machines imaging radiation is normally directed through an optical system from an original document to be copied. Conventional or even especially designed optical systems exhibit an inherent fall-off of efficiency in the optical components at their extreme fields of view. In well known automatic electrostatographic machines, it is usual to move the original to be copied, the photoconductive layer or both in synchronism during the step of imaging to provide a scanning operation. The radiation is directed through a slit or aperture which regulates the time of exposure. Present day machines often compensate for fall-off of efficiency by providing a "bow-tie" or "butterfly" type aperture in the optical system in a plane at right angles to the line of scanning.
The bow-tie aperture is narrower at its center than at its extremities and is conventionally placed near the drum surface. The size of the opening controls the duration of light exposure of the portion of the photoconductor surface passing beneath it. Thus, at its extremities it is made as wide as possible to permit the longest exposure possible to compensate for the relatively low light intensity in that area. However, the width is limited by resolution loss that accompanies a wide aperture. The product of the exposure time (aperture width) and illumination intensity defines the exposure.
To make the exposure equivalent at the center of the photoconductor where the intensity is greater, the aperture must be made narrower and thus causing reduction of the exposure time. The result is failure to utilize a good portion of the light provided by the optical system because the aperture edges intercept and absorb a portion of the light defined image pattern which would otherwise strike the photoconductive surface and form the latent image charge pattern.
Techniques other than optical have been proposed for compensation for fall-off at the extremes of the radiation pattern. British Pat. No. 1,502,146 suggests the use of a differentially charged photoconductive layer as a means for compensating for fall-off, the differential charge being created by a uniform charging step followed by a non-uniform discharge step in which the photoconductor is exposed to a non-uniform radiation source. In U.S. Pat. No. 4,072,413, description is made of the use of a corotron arranged differentially to charge the photoconductive layer such that the layer is selectively more highly charged in the central portion to compensate for differential reduction of the imaging radiation in the formation of the latent image.
Another instance where non-uniform exposure occurs is in more recently introduced laser exposed imaging systems. Unless sophisticated electronic corrections are used, the linear sweep of the laser beam is faster at the ends of each scan than at the center. In addition, the scanning system itself is less transmissive at the extremes of its sweep than at the center. Together, these losses may be as great at 50% of the center intensity. By employing a photoconductor tailored to compensate for the uneven exposure, a uniform result can be obtained.